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Heat, Moisture, Clouds and Rain

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There are two reasons why tropical weather is different from that at higher latitudes. The sun shines more directly on the tropics than on higher latitudes (at least in the average over a year), which makes the tropics warm (Stevens ). And, the vertical direction (up, as one stands on the Earth's surface) is perpendicular to the Earth's axis of rotation at the equator, while the axis of rotation and the vertical are the same at the pole; this causes the Earth's rotation to influence the atmospheric circulation more strongly at high latitudes than low. Because of these two factors, clouds and rain storms in the tropics can occur more spontaneously compared to those at higher latitudes, where they are more tightly controlled by larger-scale forces in the atmosphere. Because of these differences, clouds and rain are more difficult to forecast in the tropics than at higher latitudes. On the other hand, temperature is easily forecast in the tropics, because it doesn't change much.

The higher the temperature, the more water vapor can be in the air without condensing. As the sun shines strongly on the tropics &#; particularly on the warm oceans which have an effectively infinite amount of water to evaporate into the air &#; the overlying atmosphere becomes very humid.

Temperature and pressure both drop quickly with altitude, in the tropics as elsewhere on Earth. If air rises &#; as it can, for example, if it is a little warmer and thus lighter than the air around it &#; it will expand and cool, eventually causing some of the water vapor in it to condense into tiny liquid droplets, forming a cloud. The latent heat of the condensation warms the air, causing it to become still warmer, and allowing the updraft to rise further. If enough water condenses, the cloud droplets can become large enough to fall as rain.

Sometimes a tropical shower ends quickly, as the clouds and falling rain evaporate. The evaporating rain cools the air near the surface, so that it isn't warm enough to rise into a new cloud. Sometimes, though, the cooling effect as well as the weight of the rain itself can create a downdraft strong enough to create turbulence that in turn lifts nearby warm, humid air, making a new updraft (Figure 1). This process can feed on itself to produce a large complex of such storms that maintains rainy weather over a period of days and a region thousands of km in extent, sometimes moving coherently across the tropics and generating new storms as it moves (Mapes & Houze ). The circumstances that lead to one outcome vs. another &#; very rainy weather over a large region, completely clear skies, or anything in between &#; sometimes differ only in subtle ways.

Schematic of deep convective cloud showing an updraft, precipitation-driven downdraft, and turbulence below cloud base.

Figure 1: Schematic of deep convective cloud showing an updraft, precipitation-driven downdraft, and turbulence below cloud base.

The orientation of the tropics relative to the Earth's rotation axis also complicates tropical weather prediction. At higher latitudes, where the rotation axis doesn't make too great an angle to the vertical, the Coriolis force plays a big role in determining which way the wind blows. The Coriolis force makes the wind blow approximately parallel to the isobars, or lines of constant pressure. In the northern hemisphere, the wind goes clockwise around highs and counterclockwise around lows; opposite in the southern. If there were no Coriolis force, the wind would simply blow from high to low pressure, across the isobars. If that were to happen, the highs and lows wouldn't last long. The wind would equalize the pressure field, flowing out of highs and into lows and making the pressure horizontally uniform. By preventing this, the Coriolis force allows the highs and lows to exist for a long time. These highs and lows &#; the weather systems of the extratropics &#; evolve in a somewhat predictable way. While being pushed along by the jet stream, they often arrange themselves into alternating high-low patterns in the east-west direction, sometimes referred to as "waves". These wave patterns go through cycles of growth and decay that we understand, and that computer models are able to simulate accurately. Strong temperature contrasts &#; fronts &#; form where the highs and lows push warm air up against cold air, and clouds and rain tend to form where they push air upwards.

In the tropics, the Coriolis force is weak, the highs and lows flatten out, and global pressure maps show almost no structure &#; apart from the occasional tropical cyclone. With the flat pressure field comes a flat temperature field (see Figure 2, which shows global maps of temperature, pressure, and a measure of cloudiness, for a single day). There are no fronts in the tropics. Though temperature still drops quickly with height, at a given level the temperature is quite similar at any point within the tropics. So tropical temperature is quite predictable. Since it doesn't change much (see Figure 3, which compares a year's worth of surface temperature data from two Australian cities, one in the tropics and one in midlatitudes), it's easy to forecast, even far in advance. For a given upcoming date, just look up what the temperature was on that date in a few past years; it will probably have been not too different on that same date in different years, and will be within the same narrow range this year.

Three weather maps from May 15,

Figure 2: Three weather maps from May 15,

Top: mean sea level pressure, contour interval 5 hPa. Middle: surface temperature, contour 5&#;C. Bottom: equivalent black body temperature, K; color scale shown below the plot, values greater than are indicative of clear skies and are not shown, while lower values are indicative of clouds and rain. Note the much greater temperature and pressure variations (indicated by many more contours) in the higher latitudes compared to the tropics. Clouds and rain (as indicated by black body temperature) are nonetheless quite different from place to place within the tropics, although pressure and temperature are not.

Daily maximum surface temperatures.

Figure 3: Daily maximum surface temperatures.

Recorded at the airports in Darwin (12&#;S, &#;E) and Hobart (43&#;S, &#;E), Australia, during the year Hobart &#; the higher-latitude city &#; besides being colder overall, has much greater variations in temperature, both in the seasonal cycle and the day-to-day fluctuations, than does Darwin, the tropical city.

Winds and rain, on the other hand, are difficult to predict in the tropics. Without strong highs, lows and fronts pushing the air around and determining where it rises, the rain appears to form more from the spontaneous bubbling up of buoyant convective clouds. These convective clouds are what we know in many areas as thunderstorms &#; though over ocean in particular, they need not necessarily produce thunder and lightning. When these clouds become big and organized enough, they can generate their own large-scale weather systems. At any given moment, much of the tropics seems to have the potential for such systems to develop, but most of the time they do not, for reasons that are neither clear to scientists nor well-predicted by computer models. The humidity field may be part of the answer &#; regions of higher humidity may be more prone to disturbed weather than drier regions, and humidity varies more than temperature within the tropics, particularly in the upper atmosphere. However, this is not a completely satisfying or useful answer either, because humidity in turn is strongly influenced by the weather, and can change rapidly.

Once an organized tropical weather system does develop, it may move in a predictable way for a while. Several different types of tropical weather systems exist, each having its own typical characteristics, including its size, speed and direction of motion.

Some disturbances, for example, are known as "easterly waves" because they follow along in the direction the overall trade winds are blowing, from east to west &#; the opposite direction of the westerly winds at higher latitudes. As with the wave patterns of high and low pressure systems in the extratropics, the term wave here indicates a wavy pattern in a map of the wind field over a large area. Easterly waves have long been well known in the Caribbean, for example (Riehl ), where they arrive after forming over west Africa and moving across the tropical Atlantic ocean. Similar weather systems also occur over the tropical Pacific (Reed & Recker ) and Indian oceans. Some of them eventually strengthen into tropical cyclones &#; known as hurricanes in the Atlantic and eastern Pacific, typhoons in the western Pacific, and simply cyclones in the Indian ocean and southern hemisphere &#; which are the most organized and destructive of tropical weather systems.

Some other types of tropical weather systems were first predicted in the s by theoretical meteorologists studying the partial differential equations that govern the motion of a thin fluid layer on a rotating sphere (Matsuno ). These "equatorial waves" were then discovered to exist first in the stratosphere (the layer above that in which weather occurs, the troposphere) (Yanai & Maruyama , Wallace & Kousky ) and then later in moving cloud patterns seen from satellites, indicating their presence in the troposphere (Gruber , Zangvil , Takayabu , Wheeler & Weickmann ). These tropospheric waves are now known as convectively coupled waves (Wheeler & Kiladis ), and come in several types including Kelvin waves (which move from west to east) and mixed Rossby-gravity waves (which move from east to west, like easterly waves). These various waves are somewhat predictable, lasting for periods of days or even a week or more. Computer models still don't capture these tropical disturbances as well as they do midlatitude weather systems, but are improving. The regularity of the waves' motion by itself allows for some predictability using statistical methods (Wheeler & Weickmann ; Figure 4). These statistical forecasts essentially just assume that a given wave will continue to move in the same direction and speed as it has thus far.

Longitude-time (Hovmoeller) diagram of outgoing longwave radiation (OLR, Watts per meter squared) averaged 15&#;S&#;N, for a period in the first half of

Figure 4: Longitude-time (Hovmoeller) diagram of outgoing longwave radiation (OLR, Watts per meter squared) averaged 15&#;S&#;N, for a period in the first half of

Shading shows the total OLR anomaly, or difference from the average &#;climatological&#; conditions; blue indicates more disturbed, rainy weather while orange and red indicate drier conditions. Features tilted down to the right are moving eastward, features tilting down to the left are moving westward. The different colored contours indicate convectively coupled wave disturbances of different types: blue contours are the Madden-Julian oscillation, black are equatorial Rossby waves, green are Kelvin waves. The period after June 1 shows a forecast produced on that date by assuming propagation of these waves at their normal speeds.

&#; Nature EducationPhoto courtesy of Matthew Wheeler, Centre for Australian Weather and Climate Research. All rights reserved. View Terms of Use

The tropical climate also has slower variations that are coherent and, to some extent, predictable. The Madden-Julian oscillation (MJO) (Madden & Julian , , ) is manifest in periods of rainy weather and westerly winds, alternating with drier weather and easterly winds. The patterns move from west to east, taking a month or two to complete a cycle. The cycles are not entirely regular, and within a rainy or dry period the weather may differ from what is prevalent during that period (there may be some rain during a dry period or vice versa) so that knowing the state of the MJO by itself does not allow accurate daily weather prediction. It does, however, give some predictability to the broader state of the climate system over periods of 2&#;3 weeks or sometimes even longer. The MJO appears to be in a class by itself, as the largest and slowest of the tropical weather disturbances. Though it occurs in the tropics, the MJO also affects the weather in higher latitudes (Jones , Bond & Vecchi , Cassou ). The MJO was not predicted by the classic theory of equatorial waves &#; it was discovered first in observations, and still has not been adequately explained.

On still longer time scales, the El Ni&#;o/Southern Oscillation (ENSO) phenomenon (Philander , Sarachik & Cane , Stevens ) causes slow variations to develop in the climate of the eastern and central Pacific, with effects throughout the tropics and higher latitudes as well. This makes some aspects of the climate predictable on horizons of months to a year, though again it does not allow prediction of the specific weather on any given day that far in advance.

Tropical meteorology and climatology are active research fields. The fundamental physical mechanisms behind important phenomena, such as the Madden-Julian oscillation, easterly waves, and the formation of tropical cyclones are subjects of active research. Understanding all these phenomena will require understanding why and how deep convective clouds "self-aggregate", or merge together to form larger weather systems (Mapes , Liu & Moncrieff , Bretherton et al. ). Along with observations and theory, computer models will play an important role in untangling these mechanisms, but the models are still limited by their ability to represent the physics of convective clouds. Numerical weather prediction, which uses the same kinds of models as are used in basic scientific research, appears also to be similarly limited. We don't know how far in advance tropical weather can be predicted, even with a perfect model. The answer almost certainly depends on how precise one wishes the answer to be. For day-to-day weather at a single location, it seems likely &#; but has not been proven &#; that the intrinsic range of predictability (Lorenz ) is shorter in the tropics than higher latitudes (Boer , F. Vitart personal communication). On the other hand, if one is willing to accept forecasts of weather conditions averaged over a week, or a month, and over a larger area rather than a single location, the answer is likely to change for the better. The existence of slower tropical variations like the MJO and ENSO appears to make the tropics more predictable than the extratropics, when such averages are considered.
Sours: https://www.nature.com/scitable/knowledge/library/tropical-weather/

The tropical Atlantic has been on the active side this weekend, though the action so far has not been especially threatening or impressive. Along these lines, Tropical Storm Peter formed on Sunday morning, September 19, in the waters about miles east of the northern Leeward Islands. As of 11 a.m. EDT Sunday, Peter’s top winds were 45 mph, and the storm was moving west-northwest at 17 mph.

Peter’s current nemesis is a tropical upper tropospheric trough (TUTT), a type of upper low that can impart large amounts of shear to a nearby tropical cyclone. Strong southwesterly shear from a TUTT north of Puerto Rico has decoupled Peter, pushing most of its showers and thunderstorms (convection) well northeast of the storm’s low-level center. Some convection was beginning to form near the center on Sunday afternoon.

It’s unclear whether Peter will be able to reconsolidate. Most GFS ensemble members diminish Peter amid the strong shear, whereas the European ensemble members tend to keep Peter going, with the TUTT potentially merging with Peter.

Steering currents around the TUTT are expected to keep Peter well northeast of the Greater Antilles, and most of Peter’s disturbed weather will be on the northeast side of the storm, so any impacts to Puerto Rico and the Virgin Islands are likely to be minimal.

If Peter makes it through the next several days, it may get a chance to reintensify late in the week as it moves into more favorable conditions around the latitude of Bermuda.

TD 17 likely to become Atlantic’s next named storm

Tropical Depression 17 formed on Sunday morning in the far eastern tropical Atlantic. As of 11 a.m. EDT, TD 17 was about miles west-southwest of the Cabo Verde Islands, moving north-northwest at 14 mph. Top sustained winds were 35 mph.

With wind shear light to moderate ( knots) and sea surface temperatures warm (around 28 degrees C or 82°F), TD 17 may intensify into Tropical Storm Rose as soon as Sunday evening. The window for any development of TD 17 likely will close by midweek, as the system heads northwest and encounters cooler SSTs, higher wind shear, and a steadily drier atmosphere.

Two other systems to watch

On the heels of TD 17, another wave emerging off the African coast late Sunday could develop later in the week. This wave will move into the tropical Atlantic at a lower latitude than TD 17, potentially giving it a better chance of approaching the Caribbean in a few days. In its tropical weather outlook issued at 2 p.m. EDT Sunday, the National Hurricane Center gave this system a 40% chance of development into at least a tropical depression between Tuesday and Friday.

Meanwhile, the remnants of Tropical Storm Odette, which was declared post-tropical south of Nova Scotia on Saturday evening, could regroup later this week as a tropical or subtropical cyclone as they arc southward atop warmer waters. NHC gave 30% odds of such redevelopment between Tuesday and Friday.

Heading into uncharted territory on this year’s list of Atlantic names

According to Phil Klotzbach of Colorado State University, since accurate satellite records began in , only two other seasons have had as many as 16 named storms by September and So far in , the Atlantic has had 16 named storms, six hurricanes, and three major hurricanes. The averages were named storms, hurricanes, and major hurricanes, so by several metrics, already has had a full season of activity – and there’s over 40% of the season to go! This makes it likely that we will run through the full list of 21 names for the second year in a row.

The names remaining in are Rose, Sam, Teresa, Victor, and Wanda, none of them ever used in Atlantic tropical history. In fact, among the six rotating lists used every six years in the Atlantic since , this year’s list is the only one that has never made it to the “R” storm.

Assuming the year stays active, the 22nd named storm of would be Adria, as the season cycles through a new alphabetical list of 21 supplemental names adopted earlier this year by the World Meteorological Organization. (Naming storms after Greek letters has been permanently discontinued.)

Website visitors can comment on “Eye on the Storm” posts. Please read our Comments Policy prior to posting. Comments are generally open for 30 days from date posted. Sign up to receive email announcements of new postings here. Twitter: @DrJeffMasters and @bhensonweather

Bob Henson is a meteorologist and journalist based in Boulder, Colorado. He has written on weather and climate for the National Center for Atmospheric Research, Weather Underground, and many freelance More by Bob Henson

Jeff Masters, Ph.D., worked as a hurricane scientist with the NOAA Hurricane Hunters from After a near-fatal flight into category 5 Hurricane Hugo, he left the Hurricane Hunters to pursue a More by Jeff Masters

Sours: https://yaleclimateconnections.org//09/tropical-storm-peter-and-tdform-in-tropical-atlantic/
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Hurricane Sam Weakens Into a Post-Tropical Cyclone

The storm, which lingered over the Atlantic Ocean for more than two weeks, never posed a serious threat to land.

Hurricane Sam weakened to a post-tropical cyclone early Tuesday, after lingering over the Atlantic Ocean for nearly two weeks, the National Hurricane Center said.

As of 4 a.m. Eastern time, the storm was between Newfoundland, in eastern Canada, and Iceland, with maximum sustained winds of 80 miles per hour, forecasters said. Sam was expected to remain a powerful post-tropical cyclone over the North Atlantic on Tuesday.

Sam became the 18th named storm of the Atlantic hurricane season on Sept. 23 and later strengthened to a Category 4 storm, making it the fourth major hurricane of the year, joining Grace, Ida and Larry. The Saffir-Simpson scale classifies major hurricanes as Category 3 or higher, with maximum sustained winds above m.p.h.

The storm never posed a serious threat to land.

There is only one name, Wanda, left on this year’s primary list of 21 storm names. If more storms form, the National Weather Service will move on to a list of supplemental names, only the third time in history that it has had to do that. The first was in

“With more than two months to go in the hurricane season, it is certainly possible that the Atlantic list of names will be exhausted,” said Dennis Feltgen, a meteorologist at the hurricane center in Miami.

Last year, there were 30 named storms, including six major hurricanes, forcing meteorologists to exhaust the alphabet for the second time and move to using Greek letters. It was the most named storms on record, surpassing the 28 from , and the second-highest number of hurricanes.

The Hurricane Season So Far

Derrick Bryson Taylor🌀 Reporting on the weather

The Hurricane Season So Far

Derrick Bryson Taylor🌀 Reporting on the weather

It’s been a busy hurricane season in the Atlantic Ocean this year, and there’s still a few more months to go until it ends on Nov.

Here’s at look at the season so far →

The Hurricane Season So Far

Derrick Bryson Taylor🌀 Reporting on the weather

By late September, 20 tropical cyclones had formed in the Atlantic, some of them bringing destructive winds and torrential rains to the United States and the Caribbean.

That’s a few shy of the 23 that had formed by the same time in ’s record-breaking season. Last year eventually had 30 storms before the season’s end.

It was the second time ever that forecasters ran through their entire list of planned names.

The Hurricane Season So Far

Derrick Bryson Taylor🌀 Reporting on the weather

Among the most damaging storms so far this year was Ida, which in late August lashed New Orleans as a Category 4 hurricane, destroying property and causing power failures that lasted for weeks in some places.

The Hurricane Season So Far

Derrick Bryson Taylor🌀 Reporting on the weather

Days later, remnants of Ida swept through the Northeast, causing flooding in New York City and elsewhere. Streets and subway platforms were turned into rivers. The storm killed at least 43 people in the region.

The Hurricane Season So Far

Derrick Bryson Taylor🌀 Reporting on the weather

Elsa was another system that quickly moved up the United States after hitting the Gulf Coast. In July, Elsa made landfall southeast of Tallahassee, Fla., killing at least one person before moving into Georgia. The storm later pounded New York City, where more than a dozen people were rescued from a flooded highway in the Bronx.

Weeks later, Henri battered the Northeast, knocking out power in much of coastal Rhode Island when it came ashore as a powerful tropical storm.

The Hurricane Season So Far

Derrick Bryson Taylor🌀 Reporting on the weather

There was also Grace, which quickly followed in the Caribbean, pelting Haiti with rain days after the country was struck by a magnitude earthquake. Grace later moved over Mexico, killing at least eight.

More on hurricanes and climate change.

Item 1 of 7

In May, scientists with the National Oceanic and Atmospheric Administration forecast that there would be 13 to 20 named storms this year, six to 10 of which would be hurricanes, including three to five major hurricanes of Category 3 or higher in the Atlantic.

NOAA updated its forecast in early August, predicting 15 to 21 named storms, including seven to 10 hurricanes, by the end of the season on Nov.

The links between hurricanes and climate change are becoming more apparent. A warming planet can expect stronger hurricanes over time, and a higher incidence of the most powerful storms — though the overall number of storms could drop, because factors like stronger wind shear could keep weaker storms from forming.

Hurricanes are also becoming wetter because of more water vapor in the warmer atmosphere; scientists have suggested storms like Hurricane Harvey in produced far more rain than they would have without the human effects on climate. Also, rising sea levels are contributing to higher storm surge — the most destructive element of tropical cyclones.

Ana became the first named storm of the Atlantic season on May 23, making this the seventh year in a row that a named storm developed in the Atlantic before the official start of the season on June 1.

Johnny Diaz, Jesus Jiménez, Eduardo Medina, Vimal Patel, Chris Stanford and Daniel Victor contributed reporting.

Sours: https://www.nytimes.com/article/tropical-storm-sam-hurricane.html
Tropical update: Invests 92L, 93L and Tropical Storm Pamela (Pacific)

Heat, Moisture, Clouds and Rain

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There are two reasons why tropical weather is different from that at higher latitudes. The sun shines more directly on the tropics than on higher latitudes (at least in the average over a year), which makes the tropics warm (Stevens 2011). And, the vertical direction (up, as one stands on the Earth's surface) is perpendicular to the Earth's axis of rotation at the equator, while the axis of rotation and the vertical are the same at the pole; this causes the Earth's rotation to influence the atmospheric circulation more strongly at high latitudes than low. Because of these two factors, clouds and rain storms in the tropics can occur more spontaneously compared to those at higher latitudes, where they are more tightly controlled by larger-scale forces in the atmosphere. Because of these differences, clouds and rain are more difficult to forecast in the tropics than at higher latitudes. On the other hand, temperature is easily forecast in the tropics, because it doesn't change much.

The higher the temperature, the more water vapor can be in the air without condensing. As the sun shines strongly on the tropics — particularly on the warm oceans which have an effectively infinite amount of water to evaporate into the air — the overlying atmosphere becomes very humid.

Temperature and pressure both drop quickly with altitude, in the tropics as elsewhere on Earth. If air rises — as it can, for example, if it is a little warmer and thus lighter than the air around it — it will expand and cool, eventually causing some of the water vapor in it to condense into tiny liquid droplets, forming a cloud. The latent heat of the condensation warms the air, causing it to become still warmer, and allowing the updraft to rise further. If enough water condenses, the cloud droplets can become large enough to fall as rain.

Sometimes a tropical shower ends quickly, as the clouds and falling rain evaporate. The evaporating rain cools the air near the surface, so that it isn't warm enough to rise into a new cloud. Sometimes, though, the cooling effect as well as the weight of the rain itself can create a downdraft strong enough to create turbulence that in turn lifts nearby warm, humid air, making a new updraft (Figure 1). This process can feed on itself to produce a large complex of such storms that maintains rainy weather over a period of days and a region thousands of km in extent, sometimes moving coherently across the tropics and generating new storms as it moves (Mapes & Houze 1993). The circumstances that lead to one outcome vs. another — very rainy weather over a large region, completely clear skies, or anything in between — sometimes differ only in subtle ways.

Schematic of deep convective cloud showing an updraft, precipitation-driven downdraft, and turbulence below cloud base.

Figure 1: Schematic of deep convective cloud showing an updraft, precipitation-driven downdraft, and turbulence below cloud base.

The orientation of the tropics relative to the Earth's rotation axis also complicates tropical weather prediction. At higher latitudes, where the rotation axis doesn't make too great an angle to the vertical, the Coriolis force plays a big role in determining which way the wind blows. The Coriolis force makes the wind blow approximately parallel to the isobars, or lines of constant pressure. In the northern hemisphere, the wind goes clockwise around highs and counterclockwise around lows; opposite in the southern. If there were no Coriolis force, the wind would simply blow from high to low pressure, across the isobars. If that were to happen, the highs and lows wouldn't last long. The wind would equalize the pressure field, flowing out of highs and into lows and making the pressure horizontally uniform. By preventing this, the Coriolis force allows the highs and lows to exist for a long time. These highs and lows — the weather systems of the extratropics — evolve in a somewhat predictable way. While being pushed along by the jet stream, they often arrange themselves into alternating high-low patterns in the east-west direction, sometimes referred to as "waves". These wave patterns go through cycles of growth and decay that we understand, and that computer models are able to simulate accurately. Strong temperature contrasts — fronts — form where the highs and lows push warm air up against cold air, and clouds and rain tend to form where they push air upwards.

In the tropics, the Coriolis force is weak, the highs and lows flatten out, and global pressure maps show almost no structure — apart from the occasional tropical cyclone. With the flat pressure field comes a flat temperature field (see Figure 2, which shows global maps of temperature, pressure, and a measure of cloudiness, for a single day). There are no fronts in the tropics. Though temperature still drops quickly with height, at a given level the temperature is quite similar at any point within the tropics. So tropical temperature is quite predictable. Since it doesn't change much (see Figure 3, which compares a year's worth of surface temperature data from two Australian cities, one in the tropics and one in midlatitudes), it's easy to forecast, even far in advance. For a given upcoming date, just look up what the temperature was on that date in a few past years; it will probably have been not too different on that same date in different years, and will be within the same narrow range this year.

Three weather maps from May 15, 2005.

Figure 2: Three weather maps from May 15, 2005.

Top: mean sea level pressure, contour interval 5 hPa. Middle: surface temperature, contour 5ºC. Bottom: equivalent black body temperature, K; color scale shown below the plot, values greater than 270 are indicative of clear skies and are not shown, while lower values are indicative of clouds and rain. Note the much greater temperature and pressure variations (indicated by many more contours) in the higher latitudes compared to the tropics. Clouds and rain (as indicated by black body temperature) are nonetheless quite different from place to place within the tropics, although pressure and temperature are not.

Daily maximum surface temperatures.

Figure 3: Daily maximum surface temperatures.

Recorded at the airports in Darwin (12ºS, 131ºE) and Hobart (43ºS, 147ºE), Australia, during the year 2005. Hobart — the higher-latitude city — besides being colder overall, has much greater variations in temperature, both in the seasonal cycle and the day-to-day fluctuations, than does Darwin, the tropical city.

Winds and rain, on the other hand, are difficult to predict in the tropics. Without strong highs, lows and fronts pushing the air around and determining where it rises, the rain appears to form more from the spontaneous bubbling up of buoyant convective clouds. These convective clouds are what we know in many areas as thunderstorms — though over ocean in particular, they need not necessarily produce thunder and lightning. When these clouds become big and organized enough, they can generate their own large-scale weather systems. At any given moment, much of the tropics seems to have the potential for such systems to develop, but most of the time they do not, for reasons that are neither clear to scientists nor well-predicted by computer models. The humidity field may be part of the answer — regions of higher humidity may be more prone to disturbed weather than drier regions, and humidity varies more than temperature within the tropics, particularly in the upper atmosphere. However, this is not a completely satisfying or useful answer either, because humidity in turn is strongly influenced by the weather, and can change rapidly.

Once an organized tropical weather system does develop, it may move in a predictable way for a while. Several different types of tropical weather systems exist, each having its own typical characteristics, including its size, speed and direction of motion.

Some disturbances, for example, are known as "easterly waves" because they follow along in the direction the overall trade winds are blowing, from east to west — the opposite direction of the westerly winds at higher latitudes. As with the wave patterns of high and low pressure systems in the extratropics, the term wave here indicates a wavy pattern in a map of the wind field over a large area. Easterly waves have long been well known in the Caribbean, for example (Riehl 1954), where they arrive after forming over west Africa and moving across the tropical Atlantic ocean. Similar weather systems also occur over the tropical Pacific (Reed & Recker 1971) and Indian oceans. Some of them eventually strengthen into tropical cyclones — known as hurricanes in the Atlantic and eastern Pacific, typhoons in the western Pacific, and simply cyclones in the Indian ocean and southern hemisphere — which are the most organized and destructive of tropical weather systems.

Some other types of tropical weather systems were first predicted in the 1960s by theoretical meteorologists studying the partial differential equations that govern the motion of a thin fluid layer on a rotating sphere (Matsuno 1966). These "equatorial waves" were then discovered to exist first in the stratosphere (the layer above that in which weather occurs, the troposphere) (Yanai & Maruyama 1966, Wallace & Kousky 1968) and then later in moving cloud patterns seen from satellites, indicating their presence in the troposphere (Gruber 1974, Zangvil 1975, Takayabu 1994, Wheeler & Weickmann 2002). These tropospheric waves are now known as convectively coupled waves (Wheeler & Kiladis 1999), and come in several types including Kelvin waves (which move from west to east) and mixed Rossby-gravity waves (which move from east to west, like easterly waves). These various waves are somewhat predictable, lasting for periods of days or even a week or more. Computer models still don't capture these tropical disturbances as well as they do midlatitude weather systems, but are improving. The regularity of the waves' motion by itself allows for some predictability using statistical methods (Wheeler & Weickmann 2002; Figure 4). These statistical forecasts essentially just assume that a given wave will continue to move in the same direction and speed as it has thus far.

Longitude-time (Hovmoeller) diagram of outgoing longwave radiation (OLR, Watts per meter squared) averaged 15ºS-15ºN, for a period in the first half of 2011.

Figure 4: Longitude-time (Hovmoeller) diagram of outgoing longwave radiation (OLR, Watts per meter squared) averaged 15ºS-15ºN, for a period in the first half of 2011.

Shading shows the total OLR anomaly, or difference from the average ”climatological” conditions; blue indicates more disturbed, rainy weather while orange and red indicate drier conditions. Features tilted down to the right are moving eastward, features tilting down to the left are moving westward. The different colored contours indicate convectively coupled wave disturbances of different types: blue contours are the Madden-Julian oscillation, black are equatorial Rossby waves, green are Kelvin waves. The period after June 1 shows a forecast produced on that date by assuming propagation of these waves at their normal speeds.

© 2012 Nature EducationPhoto courtesy of Matthew Wheeler, Centre for Australian Weather and Climate Research. All rights reserved. View Terms of Use

The tropical climate also has slower variations that are coherent and, to some extent, predictable. The Madden-Julian oscillation (MJO) (Madden & Julian 1971, 1972, 1994) is manifest in periods of rainy weather and westerly winds, alternating with drier weather and easterly winds. The patterns move from west to east, taking a month or two to complete a cycle. The cycles are not entirely regular, and within a rainy or dry period the weather may differ from what is prevalent during that period (there may be some rain during a dry period or vice versa) so that knowing the state of the MJO by itself does not allow accurate daily weather prediction. It does, however, give some predictability to the broader state of the climate system over periods of 2–3 weeks or sometimes even longer. The MJO appears to be in a class by itself, as the largest and slowest of the tropical weather disturbances. Though it occurs in the tropics, the MJO also affects the weather in higher latitudes (Jones 2000, Bond & Vecchi 2003, Cassou 2008). The MJO was not predicted by the classic theory of equatorial waves — it was discovered first in observations, and still has not been adequately explained.

On still longer time scales, the El Niño/Southern Oscillation (ENSO) phenomenon (Philander 1990, Sarachik & Cane 2010, Stevens 2011) causes slow variations to develop in the climate of the eastern and central Pacific, with effects throughout the tropics and higher latitudes as well. This makes some aspects of the climate predictable on horizons of months to a year, though again it does not allow prediction of the specific weather on any given day that far in advance.

Tropical meteorology and climatology are active research fields. The fundamental physical mechanisms behind important phenomena, such as the Madden-Julian oscillation, easterly waves, and the formation of tropical cyclones are subjects of active research. Understanding all these phenomena will require understanding why and how deep convective clouds "self-aggregate", or merge together to form larger weather systems (Mapes 1993, Liu & Moncrieff 2004, Bretherton et al. 2005). Along with observations and theory, computer models will play an important role in untangling these mechanisms, but the models are still limited by their ability to represent the physics of convective clouds. Numerical weather prediction, which uses the same kinds of models as are used in basic scientific research, appears also to be similarly limited. We don't know how far in advance tropical weather can be predicted, even with a perfect model. The answer almost certainly depends on how precise one wishes the answer to be. For day-to-day weather at a single location, it seems likely — but has not been proven — that the intrinsic range of predictability (Lorenz 1969) is shorter in the tropics than higher latitudes (Boer 1995, F. Vitart personal communication). On the other hand, if one is willing to accept forecasts of weather conditions averaged over a week, or a month, and over a larger area rather than a single location, the answer is likely to change for the better. The existence of slower tropical variations like the MJO and ENSO appears to make the tropics more predictable than the extratropics, when such averages are considered.
Sours: https://www.nature.com/scitable/knowledge/library/tropical-weather-84224797/

Tropical weather r

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TERMMEANING Advisory A formal message from a Hurricane Warning Office giving warning information together with details on tropical cyclone location, intensity and movement, and precautions that should be taken. Where possible, the RSMC Miami-Hurricane Center advisory will contain a resume of all warnings in effect.   Bulletin A public release from a weather office issued in the event of the occurrence or forecast occurrence of severe weather, including the developing stage of a tropical cyclone or after formal advisories on a hurricane or tropical cyclone have been discontinued. Bulletins emphasize features which are significant for the safety of the public and summarize all warnings in effect.  Centre fix of the tropical cyclone The estimated location of the centre of a tropical cyclone.  Eye The relatively clear and calm area inside the circular wall of convective clouds, the geometric centre of which is the centre of the tropical cyclone (hurricane).    Gale and tropical storm A warning for tropical storm conditions, including warning* possible sustained winds within the range km/h ( mph) ( knots) are expected in specified areas within 24 hours or less.    Hurricane A warm core tropical cyclone in which maximum average surface wind (one-minute mean) is km/h (74 mph) (64 knots) or greater.    Hurricane season The portion of the year having a relatively high incidence of hurricanes. In the Atlantic, Caribbean and the Gulf of Mexico, it is the period from 01 June to 30 November, and in the East Pacific, from 15 May to 30 November.    Hurricane warning A warning that one or both of the following dangerous effects of a hurricane are expected in a specified area in 24 hours or less: (a) average winds km/h (74 mph) (64 knots) or higher; (b) dangerously high water or a combination of dangerously high water and exceptionally high waves, even though winds expected may be less than hurricane force.    Hurricane watch An announcement for a specific area that a hurricane or an incipient hurricane condition poses a possible threat within 36 hours.   Local action statements A public release prepared by a Weather Service Office in or near a threatened area giving specific details for its area of responsibility:
(a) weather conditions
(b) sections that should be evacuated and
(c) other precautions necessary to protect life and property.   Reconnaissance aircraft centre fix of the tropical cyclone, vortex fix The location of the centre of a tropical cyclone obtained by reconnaissance aircraft penetration.  Storm surge The difference between the actual water level under the influence of a meteorological disturbance (storm tide) and the level which would have been attained in the absence of the meteorological disturbance (i.e. astronomical tide).  Storm tide The actual sea level as influenced by a weather disturbance. The storm tide consists of the normal astronomical tide and the storm surge.  Subtropical cyclone A non-frontal low pressure system that has characteristics of both tropical and extratropical cyclones.
  • The most common type is an upper-level cold low with circulation extending to the surface layer and maximum sustained winds generally occurring at a radius of about miles or more from the centre. In comparison to tropical cyclones, such systems have a relatively broad zone of maximum winds that is located farther from the centre, and typically have a less symmetric wind field and distribution of convection.
  • A second type of subtropical cyclone is a mesoscale low originating in or near a frontolyzing zone of horizontal wind shear, with radius of maximum sustained winds generally less 30 miles. The entire circulation may initially have a diameter of less than miles. These generally short-lived systems may be either cold core or warm core.
  Subtropical depression A subtropical cyclone in which the maximum sustained surface is less than 63 km/h (39 mph) (34 knots).
Sours: https://severeweather.wmo.int/tc/cnp/acronyms.html
Tropical weather forecast: Sept 21, 2021

Tropical Storm Pamela is expected to be "near major hurricane strength" by the time it makes landfall Wednesday morning on the west-central coast of Mexico, the National Hurricane Center said Tuesday in a public advisory. The storm is expected to concoct "life-threatening" storm surge, dangerous winds, heavy rains and a threat for "significant and life-threatening flash flooding and mudslides" across affected areas. 

Mexico Tropical Weather

Located in the Pacific Ocean, Pamela is moving north and expected to continue in the general direction throughout the afternoon, followed by a faster northeastward motion Tuesday night, according to the National Hurricane Center. 

The storm has maximum sustained winds of nearly 80 miles per hour with even higher gusts. The center forecasts "steady strengthening" to occur Tuesday night into early Wednesday morning. The storm is expected to pass south of Baja California's southern tip overnight Tuesday before making landfall in Bahia Tempehuaya to Escuinapa — which is currently under a hurricane warning — in west-central Mexico.

"Pamela could be near major hurricane strength when it reaches the coast of Mexico Wednesday morning," the center said. 

Tropical storm warnings are in effect for north of Bahia Tempehuaya to Altata, south of Escuinapa to Cabo Corrientes and Islas Marias. A tropical storm watch is in effect for Los Barilles to Cabo San Lucas. 

Parts of central Texas and southeastern Oklahoma are expected to see heavy rainfall associated from the hurricane late Wednesday and Thursday, which may result in flash and urban flooding, forecasts predict. The center said an isolated maximum total of 8 inches of rain for the area is possible. 

The center advised residents in affected areas to consult their local weather offices in preparation for the potentially "life-threatening surf and rip current conditions," as well as the "large and destructive" storm surge waves which may cause coastal flooding. 

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Tori B. Powell

Tori B. Powell is a breaking news reporter at CBS News. Reach her at [email protected]

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Sours: https://www.cbsnews.com/news/hurricane-pamela-forecast-tropical-storm-path-flooding-mexico-texas/

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Tropical Storm Sam is the latest to form. Here's what we know about its potential track

The tropical storm is churning in the open Atlantic Ocean more than 1, miles east of the Leeward Islands. While it is still far away from land, impacts are possible early next week. More importantly, the storm is expected to quickly strengthen into a hurricane as it approaches the Caribbean Basin.
The official forecast shows the system becoming a hurricane on Friday and a major hurricane -- Category 3 or higher -- as early as Saturday.
"It is noteworthy that this is the 2nd earliest formation of the 18th named storm in the Atlantic basin, moving ahead of the hurricane season, and only trailing last year," says the National Hurricane Center (NHC).
Thursday's developments were an indication that this is a very busy hurricane season and that it is far from over.

Sam will be a major hurricane but the long-term path is still uncertain

"Environmental conditions all seem to be favorable for the storm to gain strength during the next several days," the NHC said in its morning discussion. "Nearly all of the models respond by showing steady strengthening during the next several days, and so does the official forecast."
The NHC cautions that the size and intensity of Sam could play a role in its ultimate track evolution.
"What is not clear at this point is what impacts it will have on land," said CNN meteorologist Dave Hennen. "Right now, it looks like it may pass north of the Leeward Islands and Puerto Rico early next week, but that could change."
Some island in the Caribbean could use the rain, including Puerto Rico, where drought has increased over the last month. Nearly 20% of the island is under moderate drought conditions, so some rain would be extremely beneficial. But, often with tropical systems, flooding could also be a concern depending on the exact track this storm takes.
"Reliable long-range computer models show that the system could eventually have direct impacts on land," says Hennen. "Places like Puerto Rico, the Virgin Islands, Bahamas, Bermuda and even the East Coast of the US need to watch the storm closely over the next week. The models don't currently agree where the storm is headed, but do agree that its likely to be a powerful hurricane."
Sours: https://www.cnn.com//09/23/weather/tropical-storm-sam-thursday/index.html


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